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  • Windows Heap Overflows

    David Litchfield

  • Windows Heap Overflows

    Introduction

    This presentation will examine how to exploit heap based buffer overflows on the Windows platform. Heap overflows have been well documented on *nix platforms, for example Matt Connover’s paper – w00w00 on heap overflows, but they’ve not been well documented on Windows, though Halvar Flake’s Third Generation Exploits paper covered the key concepts.

  • Windows Heap Overflows

    Heap based buffers are safe…. ?????

    Most developers are aware of the dangers of stack based buffer overflows but too many still believe that if a heap based buffer is overflowed it’s not too much of a problem.

    One paper on secure coding suggested that to solve the problem of stack based overflows was to move the buffer to the heap!

  • Windows Heap Overflows

    What is a heap?

    The heap is an area of memory used for storage of dynamic data. Every process has a default process heap but a developer can create their own private heaps. Space is allocated from the heap and freed when finished with.

  • Windows Heap Overflows

    Heap functions

  • Windows Heap Overflows

    Heap Design

    Each heap starts with a structure. This structure, amongst other data, contains an array of 128 LIST_ENTRY structures. Each LIST_ENTRY structure contains two pointers – see winnt.h. This array can be found at 0x178 bytes into the heap structure – call it the FreeList array.

  • Windows Heap Overflows

    Heap Design

    When a heap is first created there are two pointers that point to the first free block set in FreeList[0]. Assuming the heap base address is 0x00350000 then first available block can be found at 0x00350688.

  • Windows Heap Overflows

    Heap Design

    0x00350178 (FreeList[0].Flink) = 0x00350688 (First Free Block) 0x0035017C (FreeList[0].Blink) = 0x00350688 (First Free Block)

    0x00350688 (First Free Block) = 0x00350178 (FreeList[0]) 0x0035068C (First Free Block+4) = 0x00350178 (FreeList[0])

    When an allocation occurs these pointers are updated accordingly. As more allocations and frees occur these pointers are continually updated and in this fashion allocated blocks are tracked in a doubly linked list.

  • Windows Heap Overflows

    So where’s the problem?

    When a heap based buffer is overflowed the control information is overwritten so when the buffer (allocated block) is freed and it comes to updating the pointers in the FreeList array there’s going to be an access violation.

  • Windows Heap Overflows

    So where’s the problem?

    Example: See code listing A – heap.c

  • Windows Heap Overflows

    So where’s the problem?

    Access violation

    77F6256F mov dword ptr [ecx],eax

    77F62571 mov dword ptr [eax+4],ecx

    EAX = 0x42424242

    ECX = 0x42424242

    If we own both EAX and ECX we have an arbitrary DWORD overwrite. We can overwrite the data at any 32bit address with a 32bit value of our choosing.

  • Windows Heap Overflows

    Exploiting Heap Overflows

    Repairing the Heap Unhandled Exception Filter PEB function Pointer Vectored Exception Handling Thread Environment Block

  • Windows Heap Overflows

    Repairing the heap

    After the overflow the heap is corrupt so you’ll need to repair the heap.

    Many of the Windows API calls use the default process heap and if this is corrupt the exploit will access violate.

  • Windows Heap Overflows

    Repairing the heap

    Could repair on a per vulnerability/exploit basis. Time consuming and could run into problems.

    Need a generic way to repair the heap which is effective for all exploits. Write it once and reuse it.

  • Windows Heap Overflows

    Repairing the heap

    The best method for repairing the heap is to reset the heap making it “appear” as if it is a fresh new heap. This will keep other heap data intact but allow fresh allocations.

  • Windows Heap Overflows

    Repairing the heap

    We reset our overflow heap control structure with heap.TotalFreeSize and set the flags to 0x14 then set heap.FreeLists[0].Flink and heap.FreeLists[0].Blink to the start of the fake control structure.

    See code listing B – asm-repair-heap.

  • Windows Heap Overflows

    Exploit: Using the Unhandled Exception Filter

    The Unhandled Exception Filter method is the most common method used. The UEF is the “last ditch effort” exception handler.

  • Windows Heap Overflows

    Exploit: Using the Unhandled Exception Filter

    Location varies from OS to OS and SP to SP. Disassemble the SetUnhandledExceptionFilter function.

    77E7E5A1 mov ecx,dword ptr [esp+4]

    77E7E5A5 mov eax,[77ED73B4]

    77E7E5AA mov dword ptr ds:[77ED73B4h],ecx

    77E7E5B0 ret 4

    UEF = 0x77ED73B4

  • Windows Heap Overflows

    Exploit: Using the Unhandled Exception Filter

    When an unhandled exception occurs the following block of code is executed:

    77E93114 mov eax,[77ED73B4]

    77E93119 cmp eax,esi

    77E9311B je 77E93132

    77E9311D push edi ***

    77E9311E call eax

  • Windows Heap Overflows

    Exploit: Using the Unhandled Exception Filter

    Essence of the method is to set our own Unhandled Exception Filter.

    EDI was pushed onto the stack. 0x78 bytes past EDI is a pointer to the end of the buffer – just before the heap management control stuff.

  • Windows Heap Overflows

    Exploit: Using the Unhandled Exception Filter

    Set the UEF to an address that points to a

    CALL DWORD PTR [EDI + 0x78]

    Many can be found in netapi32.dll, user32.dll, rpcrt4.dll for example.

  • Windows Heap Overflows

    Exploit: Using the Unhandled Exception Filter

    Notes: Other OSes may not use EDI. Windows 2000 for example has a pointer at ESI+0x4C and EBP+0x74.

    Using this method you need to know the target system – i.e. what OS and what SP level.

  • Windows Heap Overflows

    Exploit: Using the Unhandled Exception Filter

    Example: See code listing C – heap-uef.c and code listing D - exploit-uef.c

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    Vectored Exception Handling is new as of Windows XP.

    Unlike traditional frame based exception handling where EXCEPTION_REGISTRATION structures are stored on the stack information about VEH is stored on the heap.

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    A pointer to the first Vectored Exception Handler is stored at 0x77FC3210. Points to a _VECTORED_EXCEPTION_NODE.

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    struct _VECTORED_EXCEPTION_NODE

    {

    DWORD m_pNextNode;

    DWORD m_pPreviousNode;

    PVOID m_pfnVectoredHandler;

    }

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    Vectored handlers are called before any frame based handlers! Technique involves overwriting the pointer to the first _VECTORED_EXCEPTION_NODE @ 0x77FC3210 with a pointer to a fake VE node.

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    77F7F49E mov esi,dword ptr ds:[77FC3210h] 77F7F4A4 jmp 77F7F4B4 77F7F4A6 lea eax,[ebp-8] 77F7F4A9 push eax 77F7F4AA call dword ptr [esi+8] 77F7F4AD cmp eax,0FFh 77F7F4B0 je 77F7F4CC 77F7F4B2 mov esi,dword ptr [esi] 77F7F4B4 cmp esi,edi 77F7F4B6 jne 77F7F4A6

    The code behind calling the vectored exception handler.

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    Need to find a pointer on the stack to our buffer. Assume it can be found at 0x0012FF50. This becomes our m_pfnVectoredHandler making the address of our pseudo _VECTORED_EXCEPTION_NODE 0x0012FF48.

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    Remember on the free we get an arbitrary DWORD overwrite:

    77F6256F mov dword ptr [ecx],eax

    77F62571 mov dword ptr [eax+4],ecx

    We set EAX to 0x77FC320C and ECX to 0x0012FF48.

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    0x77FC320C is moved into 0x0012FF48 then 0x0012FF48 is moved into 0x77FC3210 – thus our pointer is set. When an exception occurs 0x0012FF48 (our pseudo VEN) is moved into ESI and DWORD PTR[ESI+8] is called. ESI+8 is a pointer to our buffer.

  • Windows Heap Overflows

    Exploit: Using Vectored Exception Handling

    Notes: If the location of the stack (and thus the pointer to the buffer) moves this method can be unreliable.

    Example: See code listing E – heap-vector.c and F – exploit-vector.c

  • Windows Heap Overflows

    Exploit: RtlEnterCriticalSection pointer in the PEB

    Each process contains a structure known as the PROCESS ENVIRONMENT BLOCK or PEB. The PEB can be referenced from the Thread Information/Environment Block TIB/TEB. FS:[0] points to the TEB.

    mov eax, dword ptr fs:[0x30] mov eax, dword ptr fs:[eax+0x

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